[Admin][UI] Usage & Spending dashboard (+ platform-AI kill switch) #39

[Admin][UI] Usage & Spending dashboard (+ platform-AI kill switch)

Why

The visual home of the spend concern: make platform-paid AI cost unmissable and let the operator cut it off.

Scope

• Headline: 30-day platform-paid spend $ with trend.
• Breakdown tables/charts: by provider, by model, by kind (embedding/summary/translation/chat), by user (top spenders), by agent/instance. Window selector (1/7/30/90 days).
• Clear platform-paid vs BYOK visual split (the platform only pays the platform-paid slice).
• Platform-AI kill switch: a toggle bound to GET/PUT /v1/admin/settings/platform-ai, with an explicit warning that turning it off makes embeddings/summaries/translation no-op (RAG goes dark) — requires confirm; writes audit.

Acceptance criteria

• All breakdowns render from /v1/admin/usage + /v1/admin/spending.
• Platform-paid vs BYOK split is visually unambiguous.
• Kill-switch toggle reflects + changes runtime state and is audit-logged.
• Window selector re-queries; loading/empty/error states handled.

Relevant code

• store/admin/ Usage page; consumes Usage/Spending API + platform-ai settings.

References

• PAS PlatformAnalyticsPanel.tsx (SVG charts, window selector).

Part of the PAGS Admin Portal epic. Depends on UI scaffold + Usage & Spending API. Directly addresses the "platform should not pay for AI" concern.